An MS/MS analysis (tandem analysis) has been known as one of mass analyses using an ion trap mass spectrometer or similar mass analyzer. In a typical MS/MS (=MS2) analysis, ions having a specific mass-to-charge ratio m/z are first selected as precursor ions from various ions originating from a compound to be analyzed, the selected ions are dissociated through collision induced dissociation (CID) or the like, and product ions having low mass-to-charge ratios are generated. The specific form of the dissociation at this time depends on the structure of the original compound. Then, the product ions generated through the dissociation are subjected to a mass analysis, so that an MS2 spectrum is acquired. Through an analysis on the MS2 spectrum, the target compound is identified, and the chemical structure of the target compound is obtained.
However, compounds having a large molecular weight, such as peptide, and compounds difficult to dissociate cannot be dissociated into ions having an adequately low mass-to-charge ratio, through a one-stage CID operation. In such a case, selection of a precursor ion and its CID operation are repeated several times, and finally produced product ions are subjected to a mass analysis. Such a process is called the MSn analysis. Herein, mass spectrometer that performs the MSn analysis is called the MSn mass spectrometer.
In a chromatograph mass spectrometer including a liquid chromatograph (LC) or a gas chromatograph (GC) and an MSn mass spectrometer in combination, if components contained in a sample are known, the mass-to-charge ratio of precursor ions to be subjected to an MSn analysis can be set in advance for the retention time of each component in order to acquire an MSn spectrum of a target component. However, if components contained in a sample are unknown, precursor ions to be subjected to an MSn analysis cannot be set in advance, and an MSn analysis result of an unknown component that is contained in the sample in addition to a target component cannot be obtained. To deal with this, a mass spectrometer has been known up to now, the mass spectrometer having a function of: automatically selecting appropriate precursor ions from a result obtained through an MS1 analysis without a CID operation; and executing an MSn analysis in substantially real time (which is hereinafter called the automatic MSn function).
For example, as disclosed in Patent Literature 1, in a conventional MSn mass spectrometer having the automatic MSn function, examples of the conditions for automatically selecting precursor ions from among a plurality of peaks appearing on an MS1 spectrum obtained through an MS1 analysis include: selecting peaks in decreasing order of signal intensity; and selecting peaks whose signal intensity is in a predetermined intensity range. Moreover, an ion exclusion list or an ion priority list may be set in advance to prevent ions having a mass-to-charge ratio registered in the ion exclusion list from being selected as precursor ions, even if the ions satisfy the aforementioned criterion. Conversely, ions having a mass-to-charge ratio registered in the ion priority list are selected as precursor ions when peak of the ions appears, even if the ions do not satisfy the aforementioned criterion.
Particularly when protein, lipid, and the like are identified from a biological sample, it is necessary to efficiently select ions originating from a target component from among an enormous number of components including impurities to perform an MSn analysis. Accordingly, the following measurement technique has been adopted up to now. That is, precursor ions to be desired to be dissociated through CID are registered in advance in the ion priority list, and, when an ion registered in the list (which is hereinafter called the “precursor ion list”) is observed, a CID operation is executed in which the ion is set as a precursor ion.
Moreover, in the case where a multi-stage CID operation is desired to be performed to fragment ions originating from a sample component into smaller parts, a list of precursor ions to be dissociated is created for each stage of the CID operation. Specifically, for example, in the case of executing an MS3 analysis, as shown in FIG. 7, a list 333a of precursor ions to be dissociated in the first-stage CID operation and a list 333b of precursor ions to be dissociated in the second-stage CID operation are created in advance. In FIG. 7, A1, B1, etc. represent the values of mass-to-charge ratios. For example, if an ion registered in the precursor ion list 333a is detected on an MS1 spectrum during execution of an analysis, an MS2 analysis in which the detected ion is set as a precursor ion is immediately executed. If an ion registered in the precursor ion list 333b is then detected on an MS2 spectrum obtained through the MS2 analysis, an MS3 analysis in which the detected ion is set as a precursor ion for the second-stage CID operation is immediately executed, so that an MS3 spectrum is acquired.
In the example shown in FIG. 7, for example, if a peak having the mass-to-charge ratio specified by A1 is detected on the MS1 spectrum during execution of the analysis, the MS2 analysis in which the ion having the mass-to-charge ratio specified by A1 is set as the precursor ion is executed, so that an MS2 spectrum is obtained. Then, if the mass-to-charge ratio specified by any of A2, B2, and C2 is detected on the MS2 spectrum, the first-stage CID operation in which the ion having the mass-to-charge ratio specified by A1 is set as the precursor ion is performed. Subsequently, the second-stage CID operation in which the ion having the mass-to-charge ratio specified by A2, B2, or C2 is set as the precursor ion is performed. Resultant product ions are subjected to a mass analysis. As a result, the MS3 spectrum is obtained.
However, according to such a conventional automatic MSn function, actually meaningless unnecessary analyses are executed in many cases, and there is a problem of low MSn analysis efficiency. The reason for this is as follows: for example, execution of the second-stage CID operation in which an ion having the mass-to-charge ratio specified by C2 is set as the precursor ion can only be meaningful after the first-stage CID operation in which an ion having the mass-to-charge ratio specified by C1 is set as the precursor ion is executed; and, even in such a case, if an ion having a mass-to-charge ratio equal to or very close to the mass-to-charge ratio specified by C2 happen to be observed on the MS2 spectrum obtained by setting an ion having the mass-to-charge ratio specified by A1 as the precursor ion, the second-stage CID operation in which the observed ion is set as the precursor ion may be unnecessarily executed. In a chromatograph mass analysis, the sample component introduced into the mass spectrometer changes with the passage of time, and hence the time for which one sample component is introduced into the mass spectrometer is limited. Accordingly, if a substantially meaningless MS3 analysis (or an MSn analysis whose n is equal to or more than 4) is executed as described above, the time for executing an essentially necessary MS3 analysis runs short. As a result, information for identifying a component desired to be identified may be lacking, and omission of a component detection may occur in some cases.